It is known in the art of sensing aircraft speed and direction to place a pressure sensor at the end of a rotating arm and to interpret the pressure variations which occur as a result of aircraft movement through an air mass so as to provide airspeed and direction signals. See, for example, U.S. Pat. No. 4,360,888 to Onksen et al and U.S. Pat. No. 3,332,282 to D. F. Daw.
Daw's airspeed indicator is restricted to helicopters. He places a pressure sensing probe at the tip of one of the helicopter rotor blades. As the blade rotates, the amplitude of the cyclic variation of the blade tip pressure per revolution is a function of the translational speed of the helicopter. The cyclic variation is converted into an electrical signal which is passed through an inductive coupling using a pair of strategically positioned cored coils. By positioning the coils at right angles with respect to one another and in a known position with respect to the longitudinal axis of the aircraft, any component of the translational speed may be determined, and, from component measurements, the resultant translational direction with respect to the aircraft's axis may be derived. Daw's method is particularly advantageous in helicopters which do not have slip rings, many types of helicopters not being furnished therewith.
Onksen's omni-directional airspeed system calculates airspeed from a differential pressure signal indicative of the pressure difference between two rotating pitot-type sensors which are mounted at the ends of hollow tubular arms. At airspeeds other than zero, according to Onksen, the velocity of the air through the sensors varies sinusoidally, with maximum difference when the sensor arms are aligned perpendicular to the wind. At that instant the velocity of the sensor advancing into the wind is equal to the tip speed plus the airspeed, and the velocity of the wind in the sensor retreating from the wind is equal to the tip speed minus the airspeed. The resultant pressures in the two hollow tubes are then different, and the transducer outputs a voltage proportional to the differential pressure and proportional to the total speed. At the instant the arms are aligned parallel to the wind, the wind velocity in the sensors are again equal and the differential pressure transducer outputs zero. As mentioned, the resultant voltage waveform from the differential transducer is a sinusoid, according to Onksen, with amplitude related to airspeed and phase related to relative direction of the wind. Onksen et al provides a phase reference to resolve the airspeed signal into the longitudinal and lateral components.
Daw's placement of his pick-up coils automatically serves this reference function.
Both Onksen et al and Daw, in effect, assume that the pressure at the end of the rotating arm is a pure sinusoid.
Other rotating pressure sensing devices are disclosed by Beilman in U.S. Pat. Nos. 3,400,584; 3,726,139; 4,065,957 and 4,074,570.